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31717ec0ff
This enables a higher precision mode for the RTC. Signed-off-by: Rosen Penev <rosenp@gmail.com>
144 lines
4.5 KiB
Diff
144 lines
4.5 KiB
Diff
commit f94ffbc2c2a4128c4412bb483d0807722dfb682b
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Author: Russell King <rmk+kernel@armlinux.org.uk>
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Date: Fri Sep 29 11:23:31 2017 +0100
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rtc: armada38x: add support for trimming the RTC
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Add support for trimming the RTC using the offset mechanism. This RTC
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supports two modes: low update mode and high update mode. Low update
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mode has finer precision than high update mode, so we use the low mode
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where possible.
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Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
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Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
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--- a/drivers/rtc/rtc-armada38x.c
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+++ b/drivers/rtc/rtc-armada38x.c
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@@ -28,6 +28,8 @@
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#define RTC_IRQ_AL_EN BIT(0)
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#define RTC_IRQ_FREQ_EN BIT(1)
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#define RTC_IRQ_FREQ_1HZ BIT(2)
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+#define RTC_CCR 0x18
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+#define RTC_CCR_MODE BIT(15)
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#define RTC_TIME 0xC
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#define RTC_ALARM1 0x10
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@@ -343,18 +345,117 @@ static irqreturn_t armada38x_rtc_alarm_i
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return IRQ_HANDLED;
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}
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+/*
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+ * The information given in the Armada 388 functional spec is complex.
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+ * They give two different formulas for calculating the offset value,
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+ * but when considering "Offset" as an 8-bit signed integer, they both
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+ * reduce down to (we shall rename "Offset" as "val" here):
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+ *
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+ * val = (f_ideal / f_measured - 1) / resolution where f_ideal = 32768
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+ *
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+ * Converting to time, f = 1/t:
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+ * val = (t_measured / t_ideal - 1) / resolution where t_ideal = 1/32768
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+ *
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+ * => t_measured / t_ideal = val * resolution + 1
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+ *
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+ * "offset" in the RTC interface is defined as:
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+ * t = t0 * (1 + offset * 1e-9)
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+ * where t is the desired period, t0 is the measured period with a zero
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+ * offset, which is t_measured above. With t0 = t_measured and t = t_ideal,
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+ * offset = (t_ideal / t_measured - 1) / 1e-9
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+ *
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+ * => t_ideal / t_measured = offset * 1e-9 + 1
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+ *
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+ * so:
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+ *
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+ * offset * 1e-9 + 1 = 1 / (val * resolution + 1)
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+ *
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+ * We want "resolution" to be an integer, so resolution = R * 1e-9, giving
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+ * offset = 1e18 / (val * R + 1e9) - 1e9
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+ * val = (1e18 / (offset + 1e9) - 1e9) / R
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+ * with a common transformation:
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+ * f(x) = 1e18 / (x + 1e9) - 1e9
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+ * offset = f(val * R)
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+ * val = f(offset) / R
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+ *
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+ * Armada 38x supports two modes, fine mode (954ppb) and coarse mode (3815ppb).
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+ */
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+static long armada38x_ppb_convert(long ppb)
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+{
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+ long div = ppb + 1000000000L;
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+
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+ return div_s64(1000000000000000000LL + div / 2, div) - 1000000000L;
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+}
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+
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+static int armada38x_rtc_read_offset(struct device *dev, long *offset)
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+{
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+ struct armada38x_rtc *rtc = dev_get_drvdata(dev);
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+ unsigned long ccr, flags;
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+ long ppb_cor;
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+
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+ spin_lock_irqsave(&rtc->lock, flags);
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+ ccr = rtc->data->read_rtc_reg(rtc, RTC_CCR);
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+ spin_unlock_irqrestore(&rtc->lock, flags);
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+
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+ ppb_cor = (ccr & RTC_CCR_MODE ? 3815 : 954) * (s8)ccr;
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+ /* ppb_cor + 1000000000L can never be zero */
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+ *offset = armada38x_ppb_convert(ppb_cor);
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+
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+ return 0;
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+}
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+
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+static int armada38x_rtc_set_offset(struct device *dev, long offset)
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+{
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+ struct armada38x_rtc *rtc = dev_get_drvdata(dev);
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+ unsigned long ccr = 0;
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+ long ppb_cor, off;
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+
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+ /*
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+ * The maximum ppb_cor is -128 * 3815 .. 127 * 3815, but we
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+ * need to clamp the input. This equates to -484270 .. 488558.
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+ * Not only is this to stop out of range "off" but also to
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+ * avoid the division by zero in armada38x_ppb_convert().
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+ */
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+ offset = clamp(offset, -484270L, 488558L);
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+
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+ ppb_cor = armada38x_ppb_convert(offset);
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+
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+ /*
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+ * Use low update mode where possible, which gives a better
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+ * resolution of correction.
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+ */
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+ off = DIV_ROUND_CLOSEST(ppb_cor, 954);
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+ if (off > 127 || off < -128) {
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+ ccr = RTC_CCR_MODE;
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+ off = DIV_ROUND_CLOSEST(ppb_cor, 3815);
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+ }
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+
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+ /*
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+ * Armada 388 requires a bit pattern in bits 14..8 depending on
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+ * the sign bit: { 0, ~S, S, S, S, S, S }
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+ */
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+ ccr |= (off & 0x3fff) ^ 0x2000;
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+ rtc_delayed_write(ccr, rtc, RTC_CCR);
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+
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+ return 0;
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+}
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+
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static const struct rtc_class_ops armada38x_rtc_ops = {
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.read_time = armada38x_rtc_read_time,
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.set_time = armada38x_rtc_set_time,
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.read_alarm = armada38x_rtc_read_alarm,
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.set_alarm = armada38x_rtc_set_alarm,
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.alarm_irq_enable = armada38x_rtc_alarm_irq_enable,
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+ .read_offset = armada38x_rtc_read_offset,
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+ .set_offset = armada38x_rtc_set_offset,
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};
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static const struct rtc_class_ops armada38x_rtc_ops_noirq = {
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.read_time = armada38x_rtc_read_time,
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.set_time = armada38x_rtc_set_time,
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.read_alarm = armada38x_rtc_read_alarm,
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+ .read_offset = armada38x_rtc_read_offset,
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+ .set_offset = armada38x_rtc_set_offset,
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};
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static const struct armada38x_rtc_data armada38x_data = {
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